1. Field of the Invention
The invention relates generally to navigation receivers and more specifically to methods and devices for extending the battery life of battery-operated handheld navigation receivers.
2. Description of the Prior Art
When a battery-operated handheld portable global positioning system (GPS) navigation receiver is first turned on, there is an avalanche of processing demands that befall the digital signal processor (DSP) and microcomputer functions within. To begin with, a satellite acquisition mode is initiated that is charged with locking onto and tracking the satellite constellation visible overhead at the time. Although all the GPS satellites are transmitting on the same frequency, the Doppler effect caused by their velocities in orbit and the imprecise nature of local oscillators creates a spectrum of uncertainty that must be searched through by trying various receiver tunings until a signal is recognized. Such carrier frequency searching requires intense use of the DSP and microcomputer, so the faster these can process, the faster an initial lock can be obtained.
Concomitant with the carrier frequency search, the GPS receiver must also search both the code space and each code's phase possibilities. The GPS satellites each transmit a unique spread spectrum pseudo-random number (PRN) code, e.g., in order to make possible their multiple access on the same carrier frequency. The Doppler effect and the uncertainty of system time for an uninitialized GPS receiver mean that the PRN codes and their code complete phases must be searched. Knowing which GPS satellites should be in the area can cut down the search time by starting with those satellites' corresponding codes. But the code and code phase search also place intense, short-term demands on the processing powers of the DSP and microcomputer.
Once the first GPS satellite is found, others in the constellation can be found more rapidly because the carrier and GPS system time uncertainties are reduced. The list of candidate satellites to attempt tracking can also be narrowed. Nevertheless, great demands are placed on the DSP and microcomputer in the GPS receiver to complete the job of acquiring enough satellites so that a navigation fix can be computed.
When three, and preferably four GPS satellites are being tracked by the GPS receiver, it can enter in a navigation mode where the position, velocity and time can be computed. The processing demands now shift toward the microcomputer, and away from the DSP.
Microcomputers and DSP's are universally run at a constant, top speed. As a rule-of-thumb, which is especially true for CMOS-type integrated circuits, the faster the clock, the more power will be consumed. Intel's new P6 processor which is reported to clock at 200+ MHz, dissipates an incredible sixty watts that is too much for ordinary forced-air heat sink systems. Earlier processors run at clock speeds only a fraction of that, and therefore use far less power.
The popular Motorola line of microcomputers is not so easy to stop to save power and then resume. Tests indicate that restarting some Motorola microprocessors requires external wakeup services and devices to reactivate the microcomputer. It would be more useful if such microcomputers had the ability to resume their computational activity on their own initiative.
Twenty years ago, RCA Digital Integrated Circuits produced the CDP-1801 CMOS microprocessor that nominally ran at one megahertz. But because it was constructed of static CMOS, the clock could be taken down to zero (DC). At zero clock, the CDP-1801 drew only a leakage current, a few nanoamperes. The CDP-1801 was therefore easy to single-step for troubleshooting. Other digital circuits and microprocessors since have limited their clock speeds with the express purpose of limiting power consumption. As is well-known, power consumption in battery-operated portable equipment is a key concern of producers and consumers alike.
GPS receivers do eventually idle along. Once the acquisition mode has been completed and the navigation mode has been entered and the position and other housekeeping chores have been finished, the processors in the typical GPS receiver have less to do. However, conventional GPS receivers maintain a constant, usually maximum clock speed that is high enough to support the most intense high-activity periods when they arise.
What is needed is a battery-operated GPS handheld receiver that has a high clock-speed when the present processing demands require it, and then drops to a much lower clock-speed or stops the clock when there is no current processing task to perform.